1. Field of the Invention
The present invention relates to detecting light load conditions and improving light load efficiency in a switching power converter.
2. Description of the Related Arts
In switched mode power supplies, power loss results from either current conduction loss or switching loss in the power switch. Under heavy load conditions, current is high and thus conduction loss is the dominating factor. However, under light load conditions, current is low and thus switching loss is the dominating factor.
A conventional practice for improving light load efficiency in switching power converters is to reduce the switching frequency under light load conditions. In order to reduce the switching frequency under light load conditions, it is necessary to first detect such light load conditions accurately. Conventional switching power converters typically use the output signal of a control loop compensator in the switching power converters for detecting the light load conditions.
FIG. 1A conceptually illustrates a conventional flyback type AC-DC switching power converter. The power converter includes a transformer T1, diodes D1 and D2, a capacitor CO, a switch Q1, resistors R1, R2, and a controller 102. Controller 102 generates the output drive signal 110 that drives the switch Q1. The AC power is received from an AC power source (not shown) and is rectified to provide the regulated DC input voltage VIN. The input power is stored in transformer T1 while the switch Q1 is turned on because the diode D1 becomes reverse biased. The rectified AC input power is then transferred to the load L1 across the capacitor CO while the switch Q1 is turned off because the diode D1 becomes forward biased. Diode D1 functions as an output rectifier and capacitor Co functions as an output filter. The resulting regulated output voltage Vo is delivered to the load L1. Resistor RL is a pre-load that is typically used for stabilizing the output at no load conditions.
Controller 102 includes a Vout sense module 104, a loop compensation module 106, and an on-time (Ton) generator module 108. Controller 102 can employ any one of a number of well-known modulation techniques, such as pulse-width-modulation (PWM) or pulse-frequency-modulation (PFM), to control the ON and OFF states and duty cycles of power switch Q1. PWM and PFM are conventional techniques used for controlling the switching power converters by controlling the widths or frequencies, respectively, of the output drive pulse 110 driving the switch Q1 to achieve output power regulation. Thus, on-time generator 108 generates appropriate switch drive pulses 110 to control the on-times of power switch Q1 and regulate the output voltage Vo, using a feedback loop based on the sensed output voltage VO and the sensed primary side current Ip in previous switching cycles of the switching power converter.
The voltage ISENSE is used to sense the primary current Ip through the primary winding Np and switch Q1 in the form of a voltage across sense resistor Rs. On-time generator 110 uses the voltage ISENSE to implement peak current switching to limit the primary current Ip when it reaches a threshold value by generating control signal 110 to turn off the switch Q1.
The output voltage Vo is reflected across the auxiliary winding Na of transformer T1, which is input to the Vout sense module 104 of controller 102 as the voltage VSENSE via a resistive voltage divider comprised of resistors R1 and R2. Vout sense module 104 determines the feedback voltage Vfb by sampling the voltage VSENSE at the end of the transformer reset time in each switching cycle of the power converter 100. Compensation module 106 uses the feedback voltage Vfb to determine the control voltage Vc. Control voltage Vc is a parameter indicating the extent of power supply loading, i.e., how much energy has to be transferred to the load per switching cycle in order to maintain the required output voltage.
FIG. 1B illustrates an example of circuitry of the compensation module generating the control voltage Vc. Inside the compensation module 106 is a digital error generator 152 that determines the difference between digital voltage feedback value Vfb and a digital reference voltage value VREF corresponding to a target (regulated) output voltage. The resulting digital error signal 154 is input to a P-I function (proportional-integral function) 160 comprised of an integrator 164 and a proportional block 162. Integrator 164 integrates digital error signal 154 over all the previous switching cycles or a predetermined number of previous switching cycles, whose output is added to a scaled value (scaled by coefficient Kp in proportional block 162) of the digital error signal 154 in adder 166 to generate control voltage Vc. Thus, control voltage Vc indicates the extent of power supply loading, i.e., how much energy has to be transferred to the load per switching cycle in order to maintain the required output voltage. In general, an increasing control voltage Vc indicates that the output current is lower than desired and should be increased, and a decreasing control voltage Vc indicates that the output current is higher than desired and should be decreased. Although FIG. 1B illustrates a digital implementation of generating the control voltage Vc, other analog implementations can be used to generate a similar parameter reflecting the extent of power supply loading.
As can be seen from above, the control voltage Vc indicates the extent of power supply loading, i.e., the output current or load in the switching power supply. Thus, many conventional switching power supplies use this control voltage Vc as the sole parameter to detect low load conditions.
However, using the control voltage Vc to detect low load conditions may not be desirable or accurate, if the switching frequency of the switching power converter is not constant. This is because the control voltage Vc does not always correctly identify light load conditions. For example, in the flyback power supply of FIG. 1A operating in discontinuous conduction mode (DCM), the output power Pout can be expressed as:
                              P          out                =                                                            (                                                      V                                          i                      ⁢                                                                                          ⁢                      n                                                        ⁢                                      T                    on                                                  )                            2                                      2              ⁢                                                          ⁢                              L                m                                              ⁢                      f            s                    ⁢          η                                    (                  Equation          ⁢                                          ⁢          1                )            for VinTon control, or
                              P          out                =                              1            2                    ⁢                                                    L                m                            ⁡                              (                                  I                  pri_pk                                )                                      2                    ⁢                      f            s                    ⁢          η                                    (                  Equation          ⁢                                          ⁢          2                )            for primary side current control, where Pout is the output power of the switching power converter, Vin is the rectified input (line) voltage, Ton is the turn-on time of switch Q1, Lm is the magnetizing inductance of transformer T1, fs is the switching frequency, η is the conversion efficiency, Ipri—pk is the peak primary side current. The control voltage Vc determines VinTon or Ipri_pk. In both control methods, the square of the output voltage of the loop compensation module 106 (control voltage) Vc2 is proportional to the output power Pout. In conventional PWM (Pulse Width Modulation) or free-running VMS (Valley Mode Switching) control methods, high control voltage Vc represents high output power. Therefore, control voltage Vc is conventionally used to detect light load conditions. However, if there is large reduction of switching frequency fs over a short load range, higher Vc does not necessarily represents higher output power if the switching frequency changes.Furthermore, when light load conditions are detected, conventional switching power supplies may lower the switching frequencies in the same manner without regard to the level of the input line voltage. The same switching frequencies are typically used regardless of the input line voltage, or even a higher switching frequency may be used at high input line voltages. This may result in additional inefficiencies because at high input line voltages the switching loss is more significant than that at low input line voltages.